Fruiting body, a white, soon yellow, cottony layer overgrowing the surface of a host mushroom, usually a member of the Boletaceae. Three kinds of spores are produced during the life cycle, two asexual and one sexual. The sexual phase, seldom seen because the host has usually rotted beyond recognition, consists of perithecia, asci, and ascospores. The ascospores according to Rogers and Samuels are fusiform to lanceolate, unequally bicellular, 20-25 x 4-5 µm.
Two types of asexual spores are produced:

Hypomyces chrysospermus is one of a small number of fungi included here
that parasitize other fungi. The white, then yellow cottony growths which
mummify members of the Bolete family are most common in wet years, but even
then only a fraction of the available fruiting bodies are affected.
Hypomyces microspermus, which also occurs in our area is very similar but
differs in having spiny, round, asexual spores that are smaller than those
of H. chrysospermus. It should be noted that while many mushrooms fall
victim to various molds in age, only a few of these are Hypomyces species,
i.e. forming a well developed cottony layer (a subiculum) with embedded reddish-brown
perithecia.